Small biological specimens are prepared for examination under the electron microscope by placing them on a reticulate specimen grid. Specimen grids are used particularly in the field of Cryoultramicrotomy, where specimens are examined at extreme high power under supercooled (-196.degree. C.) conditions.
Conventional specimen preparation practices for Cryoultramicrotomy are illustrated by the following articles: D. Parsons, D. J. Bellotto, W. W. Schulz, M. Buja and H. K. Hagler, "Towards Routine Cryoultramicrotomy", and H. K. Hagler and L. M. Buja, "New Techniques for the Preparation of Thin Freeze Dried Cryosections For X-Ray Microanalysis", Science of Biological Specimen Preparation, pp. 161-166, both available from H. K. Hagler, University of Texas, Department of Pathology, Health Science Center at Dallas, 5323 Harry Hines Blvd., Dallas, Tex. 75235.
In conventional practice, a grid on which the microscope specimens will be mounted is coated with a formvar solution that dries to form a transparent film across the grid. The grid is then chilled to -130.degree. C. by placing it within a block inside of a cryochamber. The cryochamber is conventionally made out of stryofoam and is filled with gaseous nitrogen at -130.degree. C. Inside the cryochamber, a grid transfer arm is provided in order to move the specimen grid (which is preferably a 50 to 400 mesh copper grid) from one location to another. The transfer arm conventionally has a pair of forceps on one of its ends in order to grip the grid. At a first location, sections of tissue are placed on the grid and the transfer arm then moves the grid over to a second location. During transfer, the specimens on the open, single grid are subject to being inadvertently jarred from their ideal locations or even swept off the grid.
The grid is then placed in a formvar-coated beryllium capsule in order to retain the specimens between two films of formvar. The capsule is assembled and loaded into a coldstage, which is used to protect the cold specimen as it is transferred to the electron microscope.
Recently, a holder comprising two grids hinged to each other has been used for containing tissue specimens. Both copper mesh parts of the double grid are coated with formvar, the tissue specimens are placed on the grid and the double grid is manually folded upon itself. The use of a double grid obviates the need for an expensive beryllium capsule, as the grid itself retains the specimen between two transparent formvar films. It also protects the specimens in transfer between workstations in the cryochamber.
However, the conventional forceps and grid transfer arm are less than ideal for the transfer of the double grid from one location to another. In conventional practice, the arm forceps must grip the lower grid of the double grid, and the upper grid must be folded onto the lower grid by using a pre-cooled second pair of manual forceps. After the upper grid has been folded onto the lower grid, one of the transfer arm forceps will be wedged between the upper grid and the lower grid. The arm grid must thus be subsequently disengaged by opening the transfer arm forceps and removing the grid with another pair of manual forceps.
Either of the steps of manually folding the grid over onto the loaded lower grid or extracting the transfer arm forceps from the folded grid may disturb the location of the specimens, as these manual functions can be controlled only to the extent permitted by the motor skills of the operator.
Thus, a need has arisen for a grid securement and transfer arm which will provide a more automatic grid folding function and make the task of transferring the specimen grid from a first location to a second location surer and easier.